1. Field of the Invention
This invention relates to a low-shrinkage unsaturated polyester resin composition having good colorability and good storage stability, and more particularly to such resin composition including in specific amounts a particular unsaturated polyester, an unsaturated monomer copolymerizable with the unsaturated polyester, and polystyrene having a certain particle size, said unsaturated polyester being a reaction product of a glycol component containing certain amounts of neopentyl glycol and 2,2-bis(4-hydroxycyclohexyl) propane with a dicarboxylic acid comprising .alpha., .beta.-unsaturated dicarboxylic acid.
2. Description of the Prior Art
Generally, an unsaturated polyester exhibits a volume shrinkage of about 10% upon curing, and, even when prepared into a premixed molding compound, exhibits a mold shrinkage of about 0.2 to 0.6% (as measured in accordance with Japanese Industrial Standard K 6911). A molding material having such a mold shrinkage is not suitable to the preparation of a molding of high dimensional precision, and further, in the case of molding a middle- or large-sized no-fuse breaker housing, or performing a metal insertion-molding as in a slip ring for a motor, cracks occur in the resulting moldings due to the cure shrinkage of the molding material or the difference in coefficient of expansion between the metal and the molding material. In order to reduce the volume shrinkage of the unsaturated polyester due to curing thereof, a vinyl polymer, a vinyl polymer having an acid functional group such as a copolymer of acrylic acid with styrene or vinyl chloride, or amorphous linear saturated polyester are conventionally added as additional components to the unsaturated polyester resin. Although a desired effect is obtained in respect of volume shrinkage-reduction in any of these cases, the unsaturated polyester resin composition containing the additional components has low colorability to a colorant and the additional components are made optically ununiform upon curing to become whitish, or to come out on the surface of the resulting molding to form coloring ununiformness thereon.
To avoid the above-mentioned coloring ununiformness or unevenness, an attempt is made to use an extremely excessive amount of a pigment or dye, for example, to use, based on the resin content, about 2.0% of carbon pigment, which is 5 to 10 times as large as the percentage in the usual case. But, such an attempt results in a decrease in the electrical and mechanical characteristics of the resulting molding, or in a decrease in the storage stability of the resin composition itself or a premix thereof. It is also known that in order to prevent the coloring ununiformness, a thermoplastic resin is added as a third component to the unsaturated polyester resin, oxides or hydroxides of magnesium, calcium or the like are added thereto to chemically denature a terminal carboxyl group in the unsaturated polyester, or a graft pigment whose surface is graft-polymerized with vinyl monomer such as a graft carbon is used as an adding pigment. However, addition of the thermoplastic resin is not very effective, addition of said oxides or hydroxides causes a substantial increase in the cure shrinkage of the unsaturated polyester resin composition, and particularly in the case where this resin composition is molded into molding with heavy cross section, causes the occurrence of inner cracks, and further addition of said graft pigment fails to decrease the shrinkage of the resin composition.